It has been found desirable in superconducting magnets used, for example, in magnetic resonance imaging (MRI) to include superconducting shields surrounding the bore of the superconducting magnet. Such shields are used to confine and separate the strong magnetic fields generated within the bore of the superconducting magnet from the surrounding environment, which may be a hospital utilizing a plurality of surrounding electronic equipment which could be adversely affected by the MRI magnetic fields, and from RF pulses used in the MRI Imaging.
However, superconducting sheet material tends to be hard and/or brittle and is extremely difficult to manufacture in large sheets. Superconducting sheets such as niobium-tin (Nb.sub.3 Sn) as well as niobium-titanium (NbTi) are materials suitable for superconducting operation. However, because of manufacturing difficulties, such materials are not available in sheets having sufficiently large areas of continuous, high quality superconducting sheet adequate to surround the bore of a typical MRI superconducting magnet which commonly has circumferences in the range of 0.8-1.0 meters. Sheets of NbTi, for example, must as a result be joined together to fabricate a tubular shield large enough to surround such a magnet bore, which frequently has an axial length in the order of 2.0 meters. As a result, it becomes necessary to be able to fabricate shields which are in the order of 1 meter in diameter and 2 meters long, and which require the joining of two or more separate sheets of superconducting material to fabricate the shield. However, the joining method must maintain superconducting operation across the joint, that is maintain a current density (Jc) in the order of 3.85.times.10.sup.8 A/M.sup.2 (amps per square meter) at 2 Tesla, which is at least 22% of the starting material.
Equally important, the completed shield must be capable of superconducting operation with joints which do not interfere with, or preclude, superconducting operation and must not provide excessive heating which would require excessive cryogen to cool the shield to superconducting temperatures. Moreover, the integrated sheets notwithstanding the relatively brittle material must be able to be formed into continuous cylinders, and the resultant superconducting operation should be stable.